It is known that high voltage equipment and devices, e.g. high voltage transformers, reactors, switchgear etc., are usually equipped with bushings that are adapted for carrying current at high potential through a grounded barrier, e.g. a wall or an enclosure of the electric device such as a transformer tank.
Conventional high voltage bushings comprise a hollow tube electrical conductor. The electrical conductor connects one side of the bushing, where a high voltage electric device is connected, with the other side of the bushing where another electric device is connected. For example, when the first electric device is a transformer, the bushing is fitted on the transformer enclosure and the conductor of the bushing connects the inside of the transformer with the outside, where another electric device can be connected, e.g. a bus, surge arrester or DC-valve. The bushing further comprises a hollow insulator around the conductor made of ceramic or composite material, which is normally provided with sheds.
Electrical devices are conventionally filled with oil acting as insulating and cooling medium inside the device tank, such as a transformer tank. This means that the bushing mostly needs to be assembled to the device after the oil has been provided, which required special assembly arrangements. One example thereof is a draw rod system, which also allows on-site installation without accessing the leads from inside the transformer main tank. The lower part of the bushing's connection system is left inside the transformer before sealing. During transformer field installation, the draw rod is pushed through the bushing and connected to the bushing's connection system inside the transformer. The bushing is put in place and the bushing's connection system is pulled from the outside by the draw rod to ensure necessary contact force and low through resistance between the transformer internal contact and the bushing bottom contact.
A schematic example of a prior art general bushing will now be described with reference to FIG. 1, showing a schematic cross sectional view of a bushing 11. A high voltage conductor 2 runs through the center of a hollow bushing insulator 12 that forms a housing around the high voltage conductor.
A flange 16 is provided on the outside of the housing 12, by means of which the housing of the bushing is connected to ground, via the transformer tank wall 18.
In FIG. 1 is also shown how the bottom end portion of the high voltage conductor 2 forms a bottom contact 3 that is arranged to be connected to the internal components of the transformer.
An upper outer terminal 24 for the conductor 2 is provided at the upper end of the bushing, opposite the bottom contact end. The outer terminal 24 is electrically connected to the conductor 2 through an interface, also forming a top cover of the bushing, in order to electrically connect the conductor and thus the transformer to an external source or device.
The term high voltage is conventionally used for voltages above approximately 50 kV. Today, the upper limit in commercial high voltage devices is generally 1100 kV, but higher voltages, such as 1200 kV or even more, are envisaged. Also, current levels are increasing and may be up to 4000-5000 A or even higher.
For high voltages in the region of 500 kV and more, and current ratings of 2000 A and above, the demands on the bushings are naturally increased, e.g. when it comes to heat dissipation and cooling, electric fields, electric insulation of the bushing etc. The higher the voltage, the longer the bushing has to be, and for these high voltages the length of the bushing exceeds 10 m. In this context, it becomes essential to have a low loss and efficient cooling of the bushing. The losses in today's bushings mainly occur due to losses in the conductor and in each contact or joint in the current path between different parts of the bushing. The losses in the conductor itself can be optimized by selecting the material, the shape and the size of the conductor. It is recognized that to have an effective bushing with low losses it is important to ensure that the contact pressure between the bottom contact and the conductor is sufficient to ensure a good contact. “Cold start” of a bushing at very low temperatures with full current can be very difficult and can lead to a loss of contact pressure.
The contact pressure between the bottom contact and the conductor is ensured by applying a force on the conductor via a draw rod arrangement. The prior art draw rod arrangement is schematically shown in FIGS. 2A and 2B where a draw rod 1 is fixed to the bottom contact 3 in one end and in the other end a member 4 in mechanical contact with the top part of the conductor 2. Clamping means 5 are adapted to apply a downward force on the member 4 onto the conductor 2 to ensure that contact pressure between the bottom contact 3 and the conductor 2 is sufficient. One example of the draw rod 1 and member 4 is disclosed in EP2117016.
The draw rod is normally made of steel or another suitable metal and the conductor is made out of copper or aluminium or alloys thereof. The difference in thermal expansion between the two materials will change the contact pressure between the bottom contact and the conductor when the bushing is getting warm from electrical losses during use. There is a risk that the contact pressure will be too low and thus increasing losses or that the contact pressure will be too high and deform parts of the bushing, draw rod or contacts. To overcome this, the different solutions with springs, that are arranged to maintain the contact pressure as the temperature in the bushing changes, have been implemented. Another solution is to modify the thermal expansion of the drawing rod by using special materials that have similar thermal expansion as the conductor to minimize the difference between the change in length between the draw rod and the conductor as the temperature changes.
The solution with springs to keep up the contact pressure as the temperature in the bushing changes makes the design more complex and increases the possibility that errors occur during assembly of the bushing. As the voltage increases and the bushings get longer, the difference in length that has to be compensated with the spring increases, this makes the solution with spring more undesirable. Using special materials that have similar thermal expansion as the conductor make the design more expensive.